PostgreSQL 实现阶乘方法列举

PostgreSQL 功能庞大，对实现乘法这类运算有诸多的方法，今天我来简单列举下以下几种便捷的途径。

比如我们要计算10！

1、 可以用SQL给它展开：

t_girl=# select 1*2*3*4*5*6*7*8*9*10 as mutiply_10;
 mutiply_10
------------
    3628800
(1 row)

Time: 0.854 ms

2、 用WITH递归

t_girl=# with recursive g(m,n) as
t_girl-# (select 1 m, 10 n
t_girl(# union all
t_girl(# select m*n, (n-1) n from g where n > 1
t_girl(# )
t_girl-# select max(m) as factorial_10 from g;
 factorial_10
--------------
      3628800
(1 row)

Time: 3.893 ms

3、 用简单的函数来展开

create or replace function func_get_factorial(
f_number int
)  returns bigint
as
$ytt$
  declare i int :=1;
  declare v_result bigint := 1;
begin
  for i in 1 .. f_number loop
    v_result := v_result * i;
  end loop;
  return v_result;
end;
$ytt$
language plpgsql;

t_girl=# select func_get_factorial(10) as factorial_10;
 factorial_10
--------------
      3628800
(1 row)

Time: 1.022 ms

4、 用游标和序列函数generate_series来展开

create or replace function func_get_factorial2(
f_number int
)  returns bigint
as
$ytt$
  declare cs1 cursor for select n from generate_series(1,f_number,1) as g(n);
  declare v_result bigint := 1;
  declare v_n bigint := 0;
begin
    open cs1;
    loop
        fetch cs1 into v_n;
        exit when not found;
	v_result := v_result * v_n;
    end loop;
    close cs1;
  return v_result;
end;
$ytt$
language plpgsql;

t_girl=# select func_get_factorial2(10) factorial_10;
 factorial_10
--------------
      3628800
(1 row)

Time: 2.238 ms
t_girl=#

5、 用自定义python函数

create or replace function func_get_factorial_py(
f_number int
)  returns bigint
as
$ytt$
m = 1
n = 1
for i in range(1,f_number+1):
    m = m * i
n = m
return n
$ytt$
language plpythonu;

t_girl=# select func_get_factorial_py(10) factorial_10;
 factorial_10
--------------
      3628800
(1 row)

Time: 1.060 ms